This invention relates to methods of manufacturing ceramic articles. More particularly, the invention relates to the manufacture of low thermal expansion calcium aluminate compositions.
Low thermal expansion ceramic bodies are desirable in a wide variety of applications. For example, low expansion bodies have been used as filters for fluids, in particular, as diesel particulate filters and as substrates for catalytic converters, an example of which is known in the art as a honeycomb substrate. Additionally, low thermal expansion bodies are desirable in applications where the thermal shock resistance and the ultimate use temperature are high. Substrates used under conditions of high thermal gradients are examples of this application. Typically, structures such as honeycomb and cellular substrates are subjected to harsh environments which require high thermal shock resistance, low thermal expansion, and high mechanical shock properties. Maintaining these properties for extended periods of time in their intended environments eliminates many potentially useful refractory materials.
Cordierite substrates, typically in the form of a honeycomb body, have long been preferred for use as substrates to support catalytically active components for catalytic converters on automobiles, in part due to high thermal shock resistance of cordierite ceramics. The thermal shock resistance is inversely proportional to the coefficient of thermal expansion. That is, honeycombs with a low thermal expansion have a good thermal shock resistance and can survive the wide temperature fluctuations that are encountered in the application. Manufacturers work continuously to optimize the characteristics of cordierite substrates to enhance their utility as catalyst carriers. Specifically, manufacturers continually strive to optimize the thermal shock resistance and other properties of the cordierite substrates.
There is a need to provide alternative low CTE materials that can be used in high temperature applications and methods of making such materials.
One embodiment of the invention relates to a method of making a ceramic article comprising mixing powdered raw materials including the phases calcium monoaluminate and calcium dialuminate, forming the powders into a green ceramic body, and firing the ceramic body to a temperature such that the thermal expansion of the fired body is less than 30xc3x9710xe2x88x927/xc2x0 C. over the temperature range 25xc2x0 C. to 800xc2x0 C. In some embodiments, the firing temperature is less than about 1550xc2x0 C., and in certain embodiments, the firing temperature is less than about 1500xc2x0 C.
In certain embodiments of the method, the powdered raw materials include calcium aluminate cement. In some embodiments, the method includes adding alumina powder to the powdered raw materials. In other embodiments it may be desirable to mix the calcium aluminate cement with calcium oxide source powders such as calcium carbonate, calcium oxalate, and calcium dihydroxide powders. In other embodiments, the raw materials are provided by forming a ceramic body and firing the body to form the phases calcium monoaluminate and calcium dialuminate and grinding the body to provide powders containing calcium monoaluminate and calcium dialuminate. It may be desirable in certain embodiments to add a hydration inhibitor such as sugar or citric acid.
According to certain embodiments of the invention, dimensional control of the body is maintained during firing of the body to form the ceramic article. In some embodiments, the change in percentage change in length of the article during firing over the temperature range of about 1000xc2x0 C. and 1400xc2x0 C. is less than about 5%, preferably less than 3%, and more preferably, less than 2%.
In some embodiments of the present invention, the coefficient of thermal expansion of the article formed by the method is less than about 20xc3x9710xe2x88x927/xc2x0 C. over the temperature range 25xc2x0 C. to 800xc2x0 C. In other embodiments, the coefficient of thermal expansion of the article formed by the method is less than about 10xc3x9710xe2x88x927/xc2x0 C. over the temperature range 25xc2x0 C. to 800xc2x0 C.
certain embodiments of the invention relate to a method of manufacturing a fired calcium aluminate body comprising mixing powdered materials including phases of calcium monoaluminate and calcium dialuminate in a ratio sufficient to avoid hydration of the fired body, forming a green body from the powdered materials, and firing the body to a temperature such that the coefficient of thermal expansion is less than about 30xc3x9710xe2x88x927/xc2x0 C. over the temperature range 25xc2x0 C. to 800xc2x0 C. In certain embodiments, the amount of calcium monoaluminate present is less than about 10% by weight. In other embodiments, the amount of calcium monoaluminate present is less than about 5% by weight. In still other embodiments, the amount of calcium monoaluminate present is less than about 1% by weight.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the invention as claimed.